pci: move dereferencing of root only after verifying valid root pointer

Signed-off-by: Saravanakumar <saravanakumar.punith@gmail.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/pci/pci.c

@@ -605,13 +605,13 @@ PCIBus *pci_get_bus_devfn(int *devfnp, PCIBus *root, const char *devaddr)
     int dom, bus;
     unsigned slot;
 
-    assert(!root->parent_dev);
-
     if (!root) {
         fprintf(stderr, "No primary PCI bus\n");
         return NULL;
     }
 
+    assert(!root->parent_dev);
+
     if (!devaddr) {
         *devfnp = -1;
         return pci_find_bus_nr(root, 0);